Power Supply Rejection (PSR) is a design concern in electronic systems with a power supply distribution network. Circuits in an electronic system are typically located some distance from the system's power supply, where long lines may be used to distribute power to a collection of circuits. This type of power distribution network introduces voltage drops at the local power supply of circuits when current pulses are drawn from the power distribution line. Logic circuits commonly draw very fast current spikes from the power distribution system because they switch rapidly and drive capacitive loads. Since local power supply voltage drops disturb circuit operation, an increase in PSR is desirable.
FIG. 1 illustrates a conventional reference voltage circuit 100. In order to increase the PSR of the reference voltage circuit 100, a filter circuit 130 is coupled to and an output 111 of the reference voltage circuit 110. The input 131 of the filter circuit 130 is coupled to the output 111 of the reference voltage circuit 110 via line 120. Although the Although the filter circuit 130 suppresses noise in the desired frequency range, it does so at the expense of the reference system 100 incurring increased start-up times. Adjustments of circuit parameters in the circuit design can be made to these conventional circuits to decrease the start-up time, but these design adjustments affect the power efficiency of the reference voltage circuit. That is, a reference voltage circuit that consumes an excess amount of current may lead to a decrease in battery life in a battery-powered system, and may also require a cooling system to reduce the internal temperature of the circuit. Conventional designs, such as the one described above, that use low current, such as less than 10 uA (micro amps), may be used in slower starting reference circuits. Since, the current level restricts the start-up time, too low of current may restrict the start-up time so much that the reference voltage circuit does not power up in the desired time. For example, the startup-time may exceed 100 microseconds (us) when used in a PSRAM memory system. One type of conventional design that uses low current may be a bandgap reference circuit.
In another conventional method, the implementation of circuit changes improves the start-up time of a reference voltage circuit, but these changes are compromised by tradeoffs in performance characteristics of the circuit.
Another conventional method to increase the PSR of a reference circuit involves the addition of circuits to an existing reference voltage circuit design. This method requires not only a different circuit topology, but also additional voltage headroom for operation of the reference voltage circuit. The additional voltage headroom required for this type of method is limited by the circuit's voltage range, which is limited by the power supply.